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Abstract

The radiation of a diverse array of endemic marsupials, edentates, primates, rodents and "ungulates" has been exceedingly useful for developing a detailed biochronologic sequence of about 20 South American Land Mammal Ages (SALMAs), covering much of Cenozoic time. Independent chronologic controls on this terrestrial South American Cenozoic record have increased dramatically in the past 25 years. There now are numerous radioisotopic dates (including many new laser fusion 40Ar/39Ar analyses) and magnetochronologic studies, especially for the Neogene Period, leading to better resolution of the ages of the Riochican SALMA, the new Chilean Tinguiririca faunal interval, Deseadan SALMA, Santacrucian SALMA, "Friasian" SALMA, Colombian LaVenta sequence, Huayquerian SALMA, Ensenadan SALMA, and Uquian SALMA. 40Ar/39Ar dating and magnetostratigraphic studies are continuing actively, but current data indicate that there are significant temporal gaps in the SALMA sequence, most notably representing the early Paleocene, most of the Eocene, part of the early Oligocene, much of the early Miocene, and part of the late Miocene Epochs. In particular, the Tiupampan, Casamayoran, Mustersan, Divisaderan, and Colhuehuapian Cenozoic SALMAs (as well as the late Cretaceous [?Campanian] "Alamitian" SALMA), lack either magnetic polarity stratigraphies or radioisotopic dating, are temporally constrained only by the ages of superposed intervals or weakly justified "stage-of-evolution" arguments and thus remain relatively poorly constrained geochronologically. Best estimates for the approximate durations (question marks indicate very poor geochronologic control, and therefore highly provisional age estimates) of the SALMAs are: Lujanian, 10,000–800,000 years ago; Ensenadan, 0.8–1.2 Ma; Uquian, 1.5–3.0 Ma; Chapadmalalan, 3.4–4.0 Ma; Montehermosan, 4.0–6.8 Ma; Huayquerian, 6.8–9.0 Ma; Chasicoan, 9.0–10.0(?) Ma; Mayoan, (?)10.0–11.8 Ma; Laventan, 11.8–13.8 Ma; Colloncuran, 14.0–15.5 Ma; Friasian, uncertain; Santacrucian, 16.3–17.5 Ma; Colhuehuapian, (?)19–21(?) Ma; Deseadan, 24.5–29 Ma; New SALMA (“Tinguirirican”), 31.5–36 Ma; Divisaderan, (?)40–42(?) Ma; Mustersan, (?)45–48(?) Ma; Casamayoran, (?)51–54(?) Ma; Riochican 55.5–57 Ma; Itaboraian, 57.5–59 Ma; Peligran, 61–62.5 Ma; and Tiupampan, 63–64.5 Ma.

The South American faunal and floral record can be combined with available geochronologic information to evaluate timing and pattern of major biotic and environmental changes and events. It is clear that Cenozoic terrestrial biotas responded to both global and regional, physical and biotic, changes and events, including major plate tectonic reorganizations and associated biogeographic events (e.g., Mesozoic-early Cenozoic Gondwanan [and subsidiary North American] continental connections and biogeographic relationships, final separation of Antarctica during the ?Eocene Epoch, formation of the Isthmus of Panama in the Pliocene Epoch, etc.), global Eocene-Oligocene boundary events (including climate change, initiation of oceanic deep water flow through the Drake Passage, onset of major Antarctic ice cap formation, expansion of open habitats [wooded grasslands and grasslands], increase in hypsodont mammalian taxa, and major clade extinction, origination, and diversification), phases of Andean uplift, Pliocene-Pleistocene Epoch glaciation/climatic changes, etc.

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